The combination of lithium-ion cells in order to create large-scale batteries gives rise to a new set of safety, performance, and efficiency problems. One of the most significant problems relating to the performance and safety of the pack has to do with the fact that cells connected in series differ in capacity. Ideally, the cells should be in sync and charged optimally to their respective maximum capacity. One proposed solution to sync the cells and reach this optimized charge is described in PCT Publication No. WO/2008/137764, “Fine-Controlled Battery-Charging System”, to Sendyne Corp. et al. In addition, during discharge, the series-connected cells should all discharge to their cutoff voltage at the same time. Using today's technology, cells participating in a series connection are charged until any cell reaches its maximum capacity. Similarly, discharging and utilization of the battery pack is terminated again as soon as any participating cell reaches its cutoff voltage. It is obvious that series-connected cells that do not start at the same state of charge will also discharge out of sync, resulting in a early termination of overall battery discharge operations when additional battery capacity should have been available.
The Traditional Lithium-Ion Cell-Protection Circuit
Traditionally, lithium-ion batteries include a protection circuit for added safety. FIG. 1 illustrates the functional blocks of such a circuit. The protection circuit typically includes a voltage sensor [1] that will detect an over-voltage condition during charging, or an under-voltage condition during discharging or when in storage. A current sensor [2] monitors current through the battery cell [7] in order to prevent discharge or charge currents from exceeding specific safety values. A temperature sensor [3] is used during charging to detect cell overheating. A charge-detection circuit [4] detects the presence of a charger in order to initiate and control the charging process. All of this information is collected and processed in the system-logic module, which controls a set of switches [6] that can turn open or close the charging or discharging current path. This protection circuit is currently used as a stand-alone unit that act unsupervised in situations that threaten battery-pack safety.
In multi-cell battery packs, such as in the case of notebook-computer battery packs, protection circuits exist that can accommodate three or four cells, providing the same basic functionality as in the case of a single cell, while eliminating redundant components such as the discharge/charge FETs [6].
However, the methods such as that described above to solve this problem have thus far presented serious safety and cost drawbacks. Consequently, those proposed methods have failed to be adopted in the industry.